1. Field of the Invention
The present invention relates to a CCD (charge-coupled device) camera and particularly to a method for controlling a CCD camera which enables a CCD camera to photograph a vivid image by determining the illumination condition of a space in case of photographing a space whose illumination condition changes irregularly.
2. Description of the Background Art
Recently, as demand for digital imaging apparatus spreads, light miniature cameras are coming on the market. As an imaging sensor for such cameras, a CCD is widely used.
Most cameras have a function of automatic exposure setting or automatic aperture setting (Iris) and accordingly, are capable of photographing in the daytime and nighttime. Namely, the camera measures the illumination of the scene and in case the illumination is low, it controls the degree of exposure during imaging so that the image captured has a proper brightness. On the other hand, in case of controlling the exposure amount of an image by controlling the iris of a camera, it is difficult because the physical amount that the iris can be opened is limited, and in case the available illumination is insufficient photographing is enabled by switching the photographing mode and photographing with use of a subsidiary source of light. On the other hand, in case a subsidiary source of light is not available, the photographing operation is performed by changing the lens of a camera to one capable of photographing in the infrared ray range. Here, the case that the illumination is high means the visible ray range and photographing an image in the visible ray range means the daytime mode. Photographing the image in the infrared ray range means the nighttime mode.
Generally, the camera uses, as shown in FIG. 1, an OLPF (optical low pass filter) so as to photograph an image similarly to the sight of a person by intercepting the infrared rays on the condition of daytime mode.
FIG. 1 is a graph showing the spectral frequency characteristic of an optical low pass filter according to the conventional. The range lower than 700 nm is a visible ray range and the range higher than 700 nm is an infrared ray range. Namely, when the conversion mode of the camera is switched to a nighttime mode, the OLPF should be removed mechanically or a lens which contains the OLPF should removed manually or replaced by a dummy filter which does not have a function of absorbing infrared rays so as to photograph the image brighter. A conventional camera, as shown in FIGS. 2˜4, uses a method which determines the illumination condition by measuring the brightness of a central photographing area.
FIG. 2 illustrates a conventional photometry method. Namely, FIG. 2 shows a method in which the camera measures the illumination of a spot of an area to be photographed by a spot photometry method.
FIG. 3 illustrates another conventional photometry method. Namely, FIG. 3 shows a method for supplementing the method in which the camera measures the illumination of a spot of an area to be photographed with a spot photometry method, by measuring the illumination within an area while placing less weight on the outer portion than on the center so as to have a form of a concentric target.
FIG. 4 illustrates a further conventional photometry method. Namely, FIG. 4 shows a method for supplementing the method in which the camera measures the illumination as described above. In case of photographing a certain space, the area where the image is placed is divided into several portions, the weight of the center portion is set to be the highest while placing less weight on the outer portions as in the order of the left, right, up and down, and then the illumination is measured by obtaining the average weight from the above.
The above methods are capable of photographing an image vividly when the subjects to be photographed are all located within the central part of the photographing area, and have been implemented for a digital camcorder, personal computer camera, and a digital still camera.
On the other hand, recently, a camera is widely used for surveillance in buildings or parking lots as well as for photographing of certain subjects. Namely, in case of installing a surveillance camera in a parking lot or garage, the brightness inside the parking lot may be quite dark. Moreover, an irregular change of illumination inside the parking lot is generated as cars come in and out. With the conventional photometry method, a vivid image could not be obtained. Here, the change of illumination in the parking lot is generated by the reflection of external light into the parking lot by a certain portion of the car when the car enters a parking lot in the daytime or by the headlights of a car when the car enters the parking lot at night.
Namely, the conventional photometry methods have a disadvantage in that a certain space can not be photographed reliably, such as due to the irregularly changing illumination inside a parking lot when using such methods.